RFC 6266

Use of the Content-Disposition Header Field in the Hypertext Transfer Protocol (HTTP)

Internet Engineering Task Force (IETF) J. Reschke
Request for Comments: 6266 greenbytes
Updates: 2616 June 2011
Category: Standards Track
ISSN: 2070-1721
Use of the Content-Disposition Header Field in the
Hypertext Transfer Protocol (HTTP)
Abstract
RFC 2616 defines the Content-Disposition response header field, but
points out that it is not part of the HTTP/1.1 Standard. This
specification takes over the definition and registration of Content-
Disposition, as used in HTTP, and clarifies internationalization
aspects.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6266.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Note: This document does not apply to Content-Disposition header
fields appearing in payload bodies transmitted over HTTP, such as
when using the media type "multipart/form-data" ([RFC2388]).
2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This specification uses the augmented BNF (ABNF) notation defined in
Section 2.1 of [RFC2616], including its rules for implied linear
whitespace (LWS).
3. Conformance and Error Handling
This specification defines conformance criteria for both senders
(usually, HTTP origin servers) and recipients (usually, HTTP user
agents) of the Content-Disposition header field. An implementation
is considered conformant if it complies with all of the requirements
associated with its role.
This specification also defines certain forms of the header field
value to be invalid, using both ABNF and prose requirements
(Section 4), but it does not define special handling of these invalid
field values.
Senders MUST NOT generate Content-Disposition header fields that are
invalid.
Recipients MAY take steps to recover a usable field value from an
invalid header field, but SHOULD NOT reject the message outright,
unless this is explicitly desirable behavior (e.g., the
implementation is a validator). As such, the default handling of
invalid fields is to ignore them.
4. Header Field Definition
The Content-Disposition response header field is used to convey
additional information about how to process the response payload, and
also can be used to attach additional metadata, such as the filename
to use when saving the response payload locally.

4.2. Disposition Type
If the disposition type matches "attachment" (case-insensitively),
this indicates that the recipient should prompt the user to save the
response locally, rather than process it normally (as per its media
type).
On the other hand, if it matches "inline" (case-insensitively), this
implies default processing. Therefore, the disposition type "inline"
is only useful when it is augmented with additional parameters, such
as the filename (see below).
Unknown or unhandled disposition types SHOULD be handled by
recipients the same way as "attachment" (see also [RFC2183],
Section 2.8).
4.3. Disposition Parameter: 'Filename'
The parameters "filename" and "filename*", to be matched case-
insensitively, provide information on how to construct a filename for
storing the message payload.
Depending on the disposition type, this information might be used
right away (in the "save as..." interaction caused for the
"attachment" disposition type), or later on (for instance, when the
user decides to save the contents of the current page being
displayed).
The parameters "filename" and "filename*" differ only in that
"filename*" uses the encoding defined in [RFC5987], allowing the use
of characters not present in the ISO-8859-1 character set
([ISO-8859-1]).
Many user agent implementations predating this specification do not
understand the "filename*" parameter. Therefore, when both
"filename" and "filename*" are present in a single header field
value, recipients SHOULD pick "filename*" and ignore "filename".
This way, senders can avoid special-casing specific user agents by
sending both the more expressive "filename*" parameter, and the
"filename" parameter as fallback for legacy recipients (see Section 5
for an example).

It is essential that recipients treat the specified filename as
advisory only, and thus be very careful in extracting the desired
information. In particular:
o Recipients MUST NOT be able to write into any location other than
one to which they are specifically entitled. To illustrate the
problem, consider the consequences of being able to overwrite
well-known system locations (such as "/etc/passwd"). One strategy
to achieve this is to never trust folder name information in the
filename parameter, for instance by stripping all but the last
path segment and only considering the actual filename (where 'path
segments' are the components of the field value delimited by the
path separator characters "\" and "/").
o Many platforms do not use Internet Media Types ([RFC2046]) to hold
type information in the file system, but rely on filename
extensions instead. Trusting the server-provided file extension
could introduce a privilege escalation when the saved file is
later opened (consider ".exe"). Thus, recipients that make use of
file extensions to determine the media type MUST ensure that a
file extension is used that is safe, optimally matching the media
type of the received payload.
o Recipients SHOULD strip or replace character sequences that are
known to cause confusion both in user interfaces and in filenames,
such as control characters and leading and trailing whitespace.
o Other aspects recipients need to be aware of are names that have a
special meaning in the file system or in shell commands, such as
"." and "..", "~", "|", and also device names. Recipients SHOULD
ignore or substitute names like these.
Note: Many user agents do not properly handle the escape character
"\" when using the quoted-string form. Furthermore, some user
agents erroneously try to perform unescaping of "percent" escapes
(see Appendix C.2), and thus might misinterpret filenames
containing the percent character followed by two hex digits.
4.4. Disposition Parameter: Extensions
To enable future extensions, recipients SHOULD ignore unrecognized
parameters (see also [RFC2183], Section 2.8).

4.5. Extensibility
Note that Section 9 of [RFC2183] defines IANA registries both for
disposition types and disposition parameters. This registry is
shared by different protocols using Content-Disposition, such as MIME
and HTTP. Therefore, not all registered values may make sense in the
context of HTTP.
5. Examples
Direct the UA to show "save as" dialog, with a filename of
"example.html":
Content-Disposition: Attachment; filename=example.html
Direct the UA to behave as if the Content-Disposition header field
wasn't present, but to remember the filename "an example.html" for a
subsequent save operation:
Content-Disposition: INLINE; FILENAME= "an example.html"
Note: This uses the quoted-string form so that the space character
can be included.
Direct the UA to show "save as" dialog, with a filename containing
the Unicode character U+20AC (EURO SIGN):
Content-Disposition: attachment;
filename*= UTF-8''%e2%82%ac%20rates
Here, the encoding defined in [RFC5987] is also used to encode the
non-ISO-8859-1 character.
This example is the same as the one above, but adding the "filename"
parameter for compatibility with user agents not implementing
RFC 5987:
Content-Disposition: attachment;
filename="EURO rates";
filename*=utf-8''%e2%82%ac%20rates
Note: Those user agents that do not support the RFC 5987 encoding
ignore "filename*" when it occurs after "filename".

Appendix A. Changes from the RFC 2616 Definition
Compared to Section 19.5.1 of [RFC2616], the following normative
changes reflecting actual implementations have been made:
o According to RFC 2616, the disposition type "attachment" only
applies to content of type "application/octet-stream". This
restriction has been removed, because recipients in practice do
not check the content type, and it also discourages properly
declaring the media type.
o RFC 2616 only allows "quoted-string" for the filename parameter.
This would be an exceptional parameter syntax, and also doesn't
reflect actual use.
o The definition for the disposition type "inline" ([RFC2183],
Section 2.1) has been re-added with a suggestion for its
processing.
o This specification requires support for the extended parameter
encoding defined in [RFC5987].
Appendix B. Differences Compared to RFC 2183
Section 2 of [RFC2183] defines several additional disposition
parameters: "creation-date", "modification-date", "quoted-date-time",
and "size". The majority of user agents do not implement these;
thus, they have been omitted from this specification.
Appendix C. Alternative Approaches to Internationalization
By default, HTTP header field parameters cannot carry characters
outside the ISO-8859-1 ([ISO-8859-1]) character encoding (see
[RFC2616], Section 2.2). For the "filename" parameter, this of
course is an unacceptable restriction.
Unfortunately, user agent implementers have not managed to come up
with an interoperable approach, although the IETF Standards Track
specifies exactly one solution ([RFC2231], clarified and profiled for
HTTP in [RFC5987]).
For completeness, the sections below describe the various approaches
that have been tried, and explain how they are inferior to the
RFC 5987 encoding used in this specification.

C.1. RFC 2047 Encoding
RFC 2047 defines an encoding mechanism for header fields, but this
encoding is not supposed to be used for header field parameters --
see Section 5 of [RFC2047]:
An 'encoded-word' MUST NOT appear within a 'quoted-string'.
...
An 'encoded-word' MUST NOT be used in parameter of a MIME Content-
Type or Content-Disposition field, or in any structured field body
except within a 'comment' or 'phrase'.
In practice, some user agents implement the encoding, some do not
(exposing the encoded string to the user), and some get confused by
it.
C.2. Percent Encoding
Some user agents accept percent-encoded ([RFC3986], Section 2.1)
sequences of characters. The character encoding being used for
decoding depends on various factors, including the encoding of the
referring page, the user agent's locale, its configuration, and also
the actual value of the parameter.
In practice, this is hard to use because those user agents that do
not support it will display the escaped character sequence to the
user. For those user agents that do implement this, it is difficult
to predict what character encoding they actually expect.
C.3. Encoding Sniffing
Some user agents inspect the value (which defaults to ISO-8859-1 for
the quoted-string form) and switch to UTF-8 when it seems to be more
likely to be the correct interpretation.
As with the approaches above, this is not interoperable and,
furthermore, risks misinterpreting the actual value.

Appendix D. Advice on Generating Content-Disposition Header Fields
To successfully interoperate with existing and future user agents,
senders of the Content-Disposition header field are advised to:
o Include a "filename" parameter when US-ASCII ([US-ASCII]) is
sufficiently expressive.
o Use the 'token' form of the filename parameter only when it does
not contain disallowed characters (e.g., spaces); in such cases,
the quoted-string form should be used.
o Avoid including the percent character followed by two hexadecimal
characters (e.g., %A9) in the filename parameter, since some
existing implementations consider it to be an escape character,
while others will pass it through unchanged.
o Avoid including the "\" character in the quoted-string form of the
filename parameter, as escaping is not implemented by some user
agents, and "\" can be considered an illegal path character.
o Avoid using non-ASCII characters in the filename parameter.
Although most existing implementations will decode them as
ISO-8859-1, some will apply heuristics to detect UTF-8, and thus
might fail on certain names.
o Include a "filename*" parameter where the desired filename cannot
be expressed faithfully using the "filename" form. Note that
legacy user agents will not process this, and will fall back to
using the "filename" parameter's content.
o When a "filename*" parameter is sent, to also generate a
"filename" parameter as a fallback for user agents that do not
support the "filename*" form, if possible. This can be done by
substituting characters with US-ASCII sequences (e.g., Unicode
character point U+00E4 (LATIN SMALL LETTER A WITH DIARESIS) by
"ae"). Note that this may not be possible in some locales.
o When a "filename" parameter is included as a fallback (as per
above), "filename" should occur first, due to parsing problems in
some existing implementations.
o Use UTF-8 as the encoding of the "filename*" parameter, when
present, because at least one existing implementation only
implements that encoding.